Large electronic systems comprise many electrically interconnected components, including circuit boards and associated connector assemblies. Typically, each board carries a portion of an electronic circuit and has a plurality of terminals for making electrical contact with the terminals of other boards through the connector assemblies when the boards are inserted in the assemblies. Most systems use several nearly identically appearing boards which have different electrical characteristics, mounted in very close proximity to each other. In case of a circuit fault, the boards may be readily removed for repair or replacement and reinserted. Generally, each circuit board is designed for insertion in only one uniquely associated connector, since the insertion of a circuit board in an improper connector may cause damage both to the circuit board and the connector. Hence, a scheme for guarding against insertion of an improper circuit board, is highly desirable. In addition, when a circuit board is removed from a connector with power applied to the terminals, any resulting arcing may cause a transfer of metal between the board and connector terminals and the degradation of the electrical connection. Thus, a scheme which also prevents this arcing is even more desirable.
In the prior art, mechanical keying arrangements are often employed to ensure the correct insertion of circuit boards. In one typical mechanical keying arrangement, plug-in conducting elements are uniquely positioned on the leading edge of a board to ensure that the board is received only by the connector assembly which contains corresponding receptacle elements. This arrangement is limited by the number of conducting terminals that may be positioned on a circuit board for unique insertion in an associated connector. Another typical prior art mechanical keying arrangement involves the use of nonconducting means such as plugs and receptacles, ridges and grooves, and the like. These means may be molded into the board and connector assembly or mechanically attached to them. This arrangement is also limited in that the nonconducting means occupies space that may be utilized for additional electrical terminals. Furthermore, if at any time after initial installation a circuit board with a different keying arrangement is required in a particular mounting position, the connector assembly in the two previously mentioned prior art mechanical keying arrangements must be removed and replaced with a connector assembly having the new keying arrangement.